With the advancement of ubiquitous network society, there is a rapid increase in the demand for mobile devices such as cellular phones, notebook personal computers, and digital still cameras. As the power source for such mobile devices, it is desired to put fuel cells into practical use as early as possible since fuel cells do not have to be recharged and can continuously supply power to devices if only they get refueled.
Among fuel cells, direct oxidation fuel cells are receiving attention and studied and developed actively. Direct oxidation fuel cells generate power by using an organic fuel such as methanol or dimethyl ether, but the organic fuel is not reformed into hydrogen and is supplied directly to the anode for oxidation. The organic fuels used for direct oxidation fuel cells have high theoretical energy densities, permit easy system simplification, and can be stored easily.
A typical direct oxidation fuel cell is formed by stacking a plurality of unit cells each of which is composed of an MEA (membrane-electrode assembly) sandwiched between two separators. The MEA comprises a solid polymer electrolyte membrane having an anode or a cathode bonded to either side, and each of the anode and the cathode is composed of a catalyst layer and a diffusion layer. Each of the two separators has a flow channel, through which a fuel and water are supplied to the anode side and an oxidant is supplied to the cathode side to generate power.
The power generation process of direct oxidation fuel cells are specifically described below. For example, the electrode reactions of a direct methanol fuel cell (hereinafter referred to as a DMFC), which uses methanol as the fuel, are as follows.Anode: CH3OH+H2O→CO2+6H++6e−Cathode: (3/2)O2+6H++6e−, 3H2O
As shown above, at the anode, methanol reacts with water to produce carbon dioxide, protons, and electrons. The protons produced at the anode move to the cathode through the electrolyte membrane, and the electrons produced at the anode move to the cathode through an external circuit. At the cathode, the protons and electrons from the anode combine with oxygen to form water.
However, practical utilization of DMFCs has some problems. One of the problems relates to the problem of durability. In particular, initial deterioration of the power generating performance of DMFCs is a large problem.
The main cause of the initial deterioration of power generating performance of DMFCs is accumulation of water inside the cathode catalyst layer or at the interface between the cathode catalyst layer and the diffusion layer. The water accumulation occurs due to condensation of water produced by the reaction and water having moved from the anode with the passage of power generation time. The water accumulation due to condensation at the aforementioned interface decreases the diffusibility of oxygen and increases the cathode-side concentration overvoltage.
Further, this initial deterioration is strongly affected by methanol crossover (hereinafter referred to as MCO), which is a phenomenon in which unreacted methanol crosses through the electrolyte membrane to the cathode. This is specifically described. At the cathode catalyst layer, due to MCO, the oxidation reaction of methanol occurs together with the reduction reaction of oxygen, which is the normal electrode reaction of the cathode. Thus, when high concentration methanol is used as the fuel, an increase in the amount of MCO with the passage of power generation time causes a significant increase in cathode activation overvoltage. In addition, carbon dioxide produced by the oxidation reaction of methanol further lowers the diffusibility of oxygen. As a result, the power generation performance deteriorates significantly.
The above-described cause of the initial deterioration can be removed by supplying a large amount of air to the cathode. However, such an approach is not preferable because the use of an air pump, blower or the like in a fuel cell for supplying a large amount of air to the cathode requires more electric power for operation and upsizing of the equipment. In addition, if the amount of air supplied is excessive, the electrolyte membrane and the polymer electrolyte in the catalyst layer in the unit cell dry up, which lowers the proton conductivity. As a result, the power generating performance deteriorates significantly.
To remove the cause of the initial deterioration without causing such adverse effects, a large number of proposals have been made to improve the structure of the cathode catalyst layer itself.
For example, Japanese Laid-Open Patent Publications No. 2005-353541 and No. 2006-107877 propose providing a cathode catalyst layer with a plurality of through-holes or vertical holes so that oxygen can be smoothly supplied deep into the catalyst layer and water can be smoothly removed from the depths of the catalyst layer even when the catalyst layer is thick.
Japanese Laid-Open Patent Publication No. 2005-183368 proposes an anode catalyst layer and a cathode catalyst layer both having a thickness of 20 μm or more, wherein at least one of the catalyst layers has pores with sizes of 0.3 to 2.0 μm and the volume of these pores is equal to or greater than 4% of the volume of all the pores. In this case, a liquid fuel and air (oxygen) can easily reach the respective reaction sites inside the electrodes without lowering electronic conductivity and proton conductivity.
However, according to the aforementioned conventional techniques, it is difficult to obtain a catalyst layer with a small cathode overvoltage which allows condensed water accumulated inside the cathode catalyst layer or at the interface between the cathode catalyst layer and the diffusion layer to be efficiently removed so that oxygen diffusibility is secured for an extended period of time.
Specifically, according to the conventional techniques of Japanese Laid-Open Patent Publications No. 2005-353541 and No. 2006-107877, in an early stage of power generation in which the amount of condensed water on the cathode side is small, oxygen easily reaches the three-phase interface (electrode reaction site) through the through-holes or vertical holes present in the catalyst layer. The power generating performance thus becomes relatively good. However, with the passage of power generation time, condensed water is gradually accumulated in the through-holes or vertical holes. It is thus difficult to ensure that oxygen is supplied deep into the catalyst layer. As a result, the power generating performance sharply lowers at a certain point.
Also, the conventional technique of Japanese Laid-Open Patent Publication No. 2005-183368 merely defines the lower limit value of the thickness of the catalyst layers, the pore size, and the pore volume. It is thus difficult to say that the whole catalyst layer has an optimum pore structure in terms of all of the diffusibility of fuel and air, the removal of carbon dioxide and water which are the reaction products, electronic conductivity, and proton conductivity.
The invention has been conceived in view of the problems as described above. It is therefore an object of the invention is to provide a membrane-electrode assembly for a direct oxidation fuel cell having excellent power generating performance and durability and such a direct oxidation fuel cell, having a cathode catalyst layer with a small overvoltage which allows condensed water accumulated inside the cathode catalyst layer or at the interface between the cathode catalyst layer and the diffusion layer to be efficiently removed so that oxygen diffusibility is secured for an extended period of time.